Messages in this thread | | | Subject | Re: [PATCH RFC tip/core/rcu 05/11] rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT. | From | Steven Rostedt <> | Date | Wed, 23 Feb 2011 09:11:43 -0500 |
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On Wed, 2011-02-23 at 08:59 -0500, Mathieu Desnoyers wrote: > * Steven Rostedt (rostedt@goodmis.org) wrote: > > On Tue, 2011-02-22 at 17:39 -0800, Paul E. McKenney wrote: > > > The build will break if you change the Kconfig to allow > > > DEBUG_OBJECTS_RCU_HEAD and !PREEMPT, so document the reasoning > > > near where the breakage would occur. > > > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > > --- > > > kernel/rcupdate.c | 5 +++++ > > > 1 files changed, 5 insertions(+), 0 deletions(-) > > > > > > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c > > > index afd21d1..f3240e9 100644 > > > --- a/kernel/rcupdate.c > > > +++ b/kernel/rcupdate.c > > > @@ -214,6 +214,11 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state) > > > * Ensure that queued callbacks are all executed. > > > * If we detect that we are nested in a RCU read-side critical > > > * section, we should simply fail, otherwise we would deadlock. > > > + * Note that the machinery to reliably determine whether > > > + * or not we are in an RCU read-side critical section > > > + * exists only in the preemptible RCU implementations > > > + * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why > > > + * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT. > > > */ > > > > Shouldn't this comment also be in the kconfig where > > DEBUG_OBJECTS_RCU_HEAD is defined? > > hrm, but this is a "rcuhead_fixup_init" : it does not need to always
Looks like it was in rcuhead_fixup_free to me.
> succeed. It's just that when it is safe to recover from an error > situation, it does it. We could do: > > #ifdef CONFIG_PREEMPT > /* > * Ensure that queued callbacks are all executed. > * If we detect that we are nested in a RCU read-side > * critical > * section, we should simply fail, otherwise we would > * deadlock. > */ > if (rcu_preempt_depth() != 0 || preempt_count() != 0 || > irqs_disabled()) { > WARN_ON(1); > return 0; > } > rcu_barrier(); > rcu_barrier_sched(); > rcu_barrier_bh(); > debug_object_init(head, &rcuhead_debug_descr); > return 1; > #else > return 0; > #endif > > instead, no ?
The point is that this entire block of code is wrapped in #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD, and that config depends on PREEMPT. Thus you will never have a case where #ifdef CONFIG_PREEMPT is false.
-- Steve
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